Flame failure safeguard



Dec. 25, 1951 M O T A 2,579,884

FLAME FAILURE SAFEGUARD Filed Dec. 11, 1947 Allllllll fivnfnrs E. CRAIGTHOMSON PHILIP GlUFFRIDA fly T1] (2: 611 I Patented Dec. 25, 1951 FLAMEFAILURE SAFEGUARD E. Craig Thomson, Boston, and Philip Giufirida,

Lawrence, Mass, assignors to Combustion Control Corporation, Cambridge,Mass., a corporation of Massachusetts Application December 11, 1947,Serial No. 791,054

8 Claims.

This invention relates generally to furnace control and in particular toflame failure controls for industrial furnaces, the present inventionbeing an improvement on the device disclosed in the application of E.Craig Thomson, Serial No. 754,435, filed June 13, 1947.

The device disclosed in the Thomson applica tlon herein referred to isan electronic flame failure control in which a photocell, a flameelectrode, or both simultaneously may serve as the flame detectingelements. The object of the present improvement is to extend the lifeand utility of such a control under conditions which may occasionally beencountered when the flame rod or-the photocell and flame rodcombination is used on certain types of furnace installations. Inparticular, it has been found that on burners ignited by a high voltagespark, sharp voltage pulses may occur in the flame rod circuit becauseof the abrupt breakdown of the ignition air gap each half cycle as thevoltage on the ignition electrode rises to spark value. Such pulses areliable to shorten the life of the amplifier tube to the grid of whichthe flame rod is connected. Furthermore, the relatively large currentflowing in the flame rod circuit, because of the intense ionization ofthe flame caused by the high voltage across the ignition electrodes, mayso charge the control condenser as to give rise to an excessive timedelay in the response of the device to flame failure.

The objects of this invention are to reduce the voltage pulses acrossthe grid and cathode of the amplifier tube, to maintain the inputvoltage to the amplifier tube within safe limits under all operatingconditions likely to be encountered, to increase the reliability andadaptability of a furnace control of this type and to accomplish theseimprovements with the addition of a minimum of electrical components.

This invention can best be understood by reference to the accompanyingcircuit diagram. As explained in the Thomson application previouslyreferredto, the control is designed to energize a relay upon detectionof flame. The control system is energized by a transformer !6 havingsecondaries l9, l8, and 54, and a primary H which is connected to analternating current source at terminals 20 and 2|. The device employs asingle amplifier tube I of the pentode type having an anode ll, cathodeI2 and three control grids I3, l4 and i5, conventionally designated thecontrol screen and suppressor grids, respectively. When suppressor gridl5 V is at cathode potential, or positive with respect to cathode [2,current flows in the anode, or plate circuit, by way of taps 32 and 24of secondary l8, resistance 26, and secondary IS. The potential dropacross resistance 26 makes control grid I 3 sufliciently negative withrespect to cathode i2 so that flow of electrons to screen I4 issubstantially cut off. Relay 25, which is connected in the screencircuit, is then de-energized. The relay controls a normally opencontact 66 which is in the energizing circuits of burner motor 61, mainfuel valve 89, pilot valve 9| and primary 52 of ignition transformer 86.The relay may control additional contacts for operating other controland alarm devices. A push button 51 or other suitable device may beprovided for shunting contact 66 to permit starting of the burner. Inaddition, a push button 90 or similar device may provide for a timedelay in opening the main fuel valve.

When a negative bias is applied to suppressor I 5, current in the platecircuit is cut ofi. Control grid I 3 then assumes the potential of tap24, which, on the half cycles when the tube may conduct, is positivewith respect to the cathode. Substantial current then flows in thescreen circuit energizing relay 25. The negative bias on grid [5 may besupplied either by the photocell 30, which is arranged so as to receivelight from the main flame 39, or by the flame rod 49, which is arrangedto be in contact with pilot flame 4i, acting in conjunction with aninput network which includes resistances 28 and and condensers 29 and64. When the pilot flame is burning, for example, current flows in thecircuit including ground, flame 4|, rod 40, cable 43, the input network,tap 32, tap 24, resistance 26, and secondary [9. As is well-known, aflame has substantial rectifying properties, tending to conduct from itstip to its base more readily than in the opposite direction. A chargeis, therefore, built up on condensers 29 and 64 which is of such apolarity as to make suppressor 15 negative with respect to cathode l2.When flame' 4| becomes extinguished, breaking the flame rod circuit,this charge leaks off through resistance 28, restoring the suppressor tocathode potential and cansing relay 25 to become deenergized aspreviously suppressor 15. When light ceases to fall on the 3 photocell,the charge on condenser 29 leaks 05 through resistance 28.

The device may be used with the flame rod alone, the photocell alone, orwith both the flame rod and the photocell connected as shown. In anycase, a certain amount'of alternating current, arising from such factorsas incomplete rectification in the flame and external leakage across therectifying elements, ordinarily flows in the input circuit. Thisalternating current gives rise to an alternating potential across theinput circuit which, on the half cycles when the tube may conduct,opposes the bias produced on suppressor l5 by the charge accumulatedoncondenser 29 as a result of the direct current component in the inputcircuits. Under normal conditions, the alternating component isrelatively small and suppressor [5 remains negative with respect to thecathode whenever flame is detected by either detecting element. Underabnormal conditions, however, if the alternating current resistance ofthe. flame rod should be substantially lowered, for example, by leakagedue to accumulation of soot across the insulating sleeve 85, thealternating potential becomes large enough. to overcome the negativebias of suppressor F5 on the half cycles when the tube may conduct,Current then flows in the plate circuit, and current in the screencirsuit is substantially cut off. Relay 25, therefore,

becomes de-energized on the occurrence of exces- 1 sive leakage in theflame, rod circuit.

A similar action occurs if; leakage across the photocell becomesexcessive as a result of moisture accumulating on the'photocell base, orof a conductive deposit accumulating on the interior surface of thephotocell envelope. It will be noted that photocell 3!! normallyconducts so as to charge condenser 29 on the half cycle when tube isnon-conductive. If leakage across the phototube has become excessive,the charge on condenser 29 is largely dissipated during the half cyclewhen tube in may conduct and the negative bias is removed fromsuppressorlS. "This action is generally knownlas a "safe failure and isof utmost importance in furnace safety control.

The impedance of condenser 64 is sufficiently high so thatthealternating potential across the flame rod; input circuit, whichconsists of the parallel network of condenser 64 and resistance 28, isnot shifted in phase enough to disturb the function of the circuit undertheexcess leakage condition. Representative values are .001 microfaradfor condenser 64 and 150 megohms for resistance 28. V v In a device ofthis type, it is usually desirable to introduce a certain time delay ofthe order of three or four secondsin the response to prevent falseresponses due to flickering of the flame. Since the capacity ofcondenser 64 is too small to provide this delay, a second condenser 29of e o der of m cr fared s. co nec ed in a allel with resistancezil.This condenser serves both as a, second input condenser for the flamerod circuit, and'as the input condenser'for the photocell circuit. 'Itis apparent that condenser 29. is of such high capacity that thedifferential in capacitance introduced 'by condenser 64 will notappreciably alter the time delay, and the delay is nearly the same forthe flame rod and photocell circuits. In series with condenser 29 is aresistance 6501 the order of .20 megohms which makes the impedance ofthis branch of the circuitv high enough so as not'to afiect appre ciablythe operation of the flame rod input network, consisting of 'comienseriM and resistance 28, under the excess leakage condition. Sinceresistance 65 is low as compared to resistance 28, resistance 65 may bevaried without appreciably affecting the time delay. Variation of eithercondenser 54 or resistance 28, however, will shift the phase of thealternating potential applied to suppressor I5, and the value of theleakage resistance across the flame rod which will cause relay 25 todrop out may therefore be varied by either of these means substantiallyindependently of the time delay.

The high voltage spark ignition used to ignite many types of furnaces isliable to cause intense ionization of the flame. Under this conditionthe direct current flowing in the flame rod circuit may becomeabnormally high, charging condenser 29 to a potential which would resultin an excessive time delay in the response in case of flame failure, andapplying a bias voltage to suppressor [5 which may be so high as to bedestructive to. the tube.

To limit the voltage applied to suppressor 15 by the flame rod circuitto a safe value under all conditions, a, glow tube 63 is connectedbetween suppressor l5 and cathode I2. This tube is of the two electrodetype filled with a gas, such as neon, under low pressure. The tuberemains substantially non-conductive until the voltage across it reachesthe ionization potential of the gas, for example, about volts.Thereafter, the tube conducts at substantially constant voltage withincreasing current. The presence of tube 63 has no effect on theoperation of the circuit under normal operating conditions, when thevoltage between the cathode l2 and the suppressor [5 re mains within asafe range of values. If the direct current in the flame rod circuitbecomes unusually high, as it may during the ignition period, the tube63 starts to conduct and prevents further increase in the voltagebetween suppressor and cathode.

In place of the glow tube, a resistance made of a suitable material suchas-thyrite which has a non-linear resistance characteristic, offering ahigh resistance to the relatively low voltages at which the inputcircuit normally operates and dropping sharply in resistance above acertain voltage, may be used. From the foregoing description, it isapparent that this circuit, while retaining all the safe fail.- urefeatures and other advantages described in the Thomson applicationherein referred to, provides for safe andsatisfactory operation of thedevice under the conditions which may be en-: countered at the start-ofthe operating'cycle of certain types of furnaces. Also disclosed hereinis a simple and eflicient method of adjusting the sensitivity of thedevice with respect to leakage across the detecting element withoutdisturbing other operating characteristics. It will be further apparentthat these advantages have been achieved by the addition of only a fewsimple 61B: ments in the circuit. Since certain changes may be made inthe above-described article and different embodiments of the inventioncould be made without departing from the scope thereof, it is intendedthat all matter contained in the above description or shown in theaccompanying drawing shall be interpreted as illustrative only and notin a limiting sense.

What is claimed is:

1. In a flame failure control employing a dc: testing element adapted toform an electrical contact with the flame: a discharge device having ananode, cathode and control electrode; an input circuit including saidcathode, said control electrode, said detecting element, and a networkcomprising a first capacitor, a first resistance connected in paralleltherewith, and, also in parallel therewith, a second resistanceconnected in series with a second capacitance; and a gas filled diode ofthe voltage regulating type connected in parallel with said network.

2. A control device comprising: a discharge device having a first and asecond current path and a control electrode adapted, upon applicationthereto of a control potential, to shift current from one to the otherof said paths; a load in circuit with one of said paths; a flamedetecting element; an input circuit for applying said control potentialincluding said flame detecting element, said cathode, said controlelectrode, and a network comprising a first resistance, a firstcapacitance in parallel therewith, and, also in parallel therewith, asecond resistance in series with a second capacitance; and a diode ofthe voltage regulating type connected in parallel with said network.

3. A device according to claim 2, said flame detecting element being anelectrode disposed so as to make electrical contact with a flame.

4. A device according to claim 3 having a second flame detecting elementconnected in series with said second capacitance.

5. A flame failure control device comprising: a flame detecting element;a discharge device having an anode, cathode and control electrode; aninput circuit including said cathode, said control electrode,saiddetecting element, and a network comprising a f rst capacitor, 9.first resistance connected in parallel therewith, and, also in paralleltherewith, a second resistance connected in series with a secondcapacitance; and a diode of the voltage regulating type connected inparallel with said network.

6. A device according to claim 5, said flame detecting element being anelectrode disposed so as to make electrical contact with a flame.

7. A device according to claim 5, having a second flame detectingelement connected in series with said second capacitance.

8. A device according to claim 7, said second flame detecting elementbeing a photocell.

E. CRAIG THOMSON. PHILIP GIUFFRIDA.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

